Closely-spaced, independently-addressable, solid state laser arrays are important devices in current high-speed xerographic printing technology. Generally, in xerographic printing, an electrostatic image (also known as a latent image)is formed on a photoreceptor before producing a permanent image on a recording medium such as paper. To form a latent image, the entire surface of the photoreceptor is first uniformly, electrostatically, charged. Then, the photoreceptor is illuminated in an image-wise manner in order to discharge selected (image or non-image) areas. One common illuminator is a modulated laser beam which is directed onto a scanning device, such as is a rotating, multi-faceted, mirrored polygon which deflects the laser light onto selected areas of the photoreceptor. Rotating polygon mirrors and their related optics are generically referred to as ROSs (Raster Output Scanners). The areas exposed to the laser beam are discharged, resulting in a pattern charged and discharged areas on the photoreceptor. This pattern, which is the latent image, is then made visible (i.e. developed) by bringing very fine toner particles into contact with the photoreceptor. The toner adheres to selected areas of the photoreceptor and is then transferred to a recording medium.
Full-color printing is usually accomplished by forming and developing four latent images, as described above, and then transferring the individual developed images in registration on a recording medium. The photoreceptor is sequentially scanned four times by the ROS, with image information for black and color separations for each of the three primary colors.
By utilizing multiple laser beams, as described in U.S. Pat. No. 5,243,359 entitled "Raster Output Scanner For A Multistation Xerographic Printing System" by T. Fisli, four laser beams, the speed of color xerographic printing may be greatly increased. As disclosed, a multiple wavelength, multi-laser array of single discrete lasers is utilized; one for each color separation and black. The four laser beams simultaneously strike a single raster output polygon mirror and a single set of scan optics, and are then separated by optical filters, and each beam is directed to a separate photoreceptor for printing one color. Then the separate images are transferred to a suitable record medium. Of course, registration of the color separations is imperative. In order to ensure that the color image produced by each photoreceptor is in registration, the laser beams should ideally originate from a common spatial location so that they share a common optical axis with respect to the rotating polygon mirror. Therefore, in practice, the laser beams should be as close to one another as possible.
It is also important that the emitting regions in the monolithic laser array structure are individually addressable and that the emitted, closely spaced beams may be individually detected and processed. Thus, any information contained as a result of beam modulation at the source or reflected by transmission through the optical path should be readily captured. There are several ways to generate beams with unique characteristics that will make the beams easier to separate or detect. One way is to change the wavelengths; a second way is to change the polarization. In both cases, there must be sufficient separation so that the optical filters can effectively isolate each beam.
For individual edge emitting lasers in a monolithic array to have independently separable wavelengths, the difference in wavelength between any two lasers generally has to be on the order of at least 50 nanometers (nm). As discussed previously, since there are four colors for full-color printing, four laser beams with wavelengths that span at least 150 nm would be required to implement this approach. To make a four-laser monolithic array with such a span of wavelengths, the use of two semiconductor material systems is generally required. For instance, an Al.sub.x Ga.sub.1-x As material system produces lasers with wavelengths from approximately 750 nm to 850 nm while an AlGaInP material system produces lasers from approximately 630 to 700 nm. By incorporating both material systems, it is possible to produce an array with a wavelength separation of at least 150 nm. Of course, the processing of dual material system arrays is more complex and expensive than the processing of single material system arrays.
Where four discrete and separable lasers are needed it is possible to combine several wavelengths and several polarizations in a single array. For example, in U.S. Pat. No. 5,465,263, entitled "Monolithic Multiple Wavelength Dual Polarization Laser Diode Arrays" by Bour et al, there is disclosed a linear monolithic multiple wavelength, dual polarization edge emitting laser array. Thus, in a four laser array, as needed in a four color system, the wavelength separation required is reduced from 150 nm to 50 nm, since the number of separable sources is doubled by taking advantage of the orthogonal polarizations.
Although edge emitting laser arrays are currently used in the vast majority of applications, vertical cavity surface-emitting lasers ("VCSELs") offer several advantages over edge emitting laser arrays. For instance, (1) VCSELs can be fabricated in closer packed arrays than edge emitting lasers; (2) the beam of a VCSEL has a small and circular angular divergence; (3) a VCSEL's mirrors are monolithically incorporated into its design, thus allowing for on wafer testing, whereas edge emitting lasers are not complete until the wafer is cleaved into individual devices; and (4) VCSELs can be monolithically grown in one-dimensional or two-dimensional arrays, whereas monolithic edge emitting lasers can only be grown in one dimensional arrays.
Therefore, it is an object of the present invention to provide a two-dimensional monolithic array of VCSEL lasers which can operate efficiently at several wavelengths with controllable linear polarizations.